An important subject for those engaged in the preparation of emulsions is to increase the covering power of emulsion silver. The reason for this is that if the covering power is increased, the amount of silver necessary to obtain a given optical density can be reduced.
The addition of various additives to the silver halide photographic emulsion to increase the covering power of emulsion silver is known. Most of these additives as covering power-increasing agents are synthetic polymers or saccharides. A typical example of a synthetic polymer is polyacrylamide, which is described in, for example U.S. Pat. Nos. 3,271,158, and 3,514,289. A typical example of a saccharide is dextran, which is described in, for example, U.S. Pat. Nos. 3,063,838 and 3,272,631.
However, if the above covering power-increasing agents are used in an amount to fully attain the object, the following problems arise during development.
(1) The emulsion layer peels apart from the subbing layer (the layer provided to adhere together the emulsion layer and a support) during development.
(2) The mechanical strength of the emulsion layer is seriously decreased and the layer tends to be scratched.
(3) Particularly where the covering power-increasing agent is added to an X-ray-sensitive material, dot formed fog, i.e., the so-called roller marks, are formed after development using an automatic developing machine.
As can be easily understood, light-sensitive materials with the above problems have low practical value.